A. To determine the site of the PR interval prolongation associated with aging, we performed signal averaged high resolution surface ECGs in 161 clinically healthy Baltimore Longitudinal Study of Aging (BLSA) volunteers with normal atrioventricular (AV) conduction. An increase in PR interval with age was found in both sexes and was localized proximal to the His bundle depolarization but distal to the P wave inscription, suggesting block within the AV junction; a similar qualitatively similar but more pronounced delay was noted proximal to the His bundle in 7 older men with first degree AV block. B. We have determined the prevalence and significance of exercise-induced frequent or repetitive ventricular ectopic beats (VEB) in apparently healthy BLSA volunteers. Between 1974 and 1984, 8O of 1160 such asymptomatic subjects developed frequent VEB (> 10%) or salvos (> 3 in a row) on at least one maximal treadmill exercise test. These 80 subjects were significantly older than the larger group without such exercise-induced VEB (63.8 +/- 12.5 vs 50.0 +/- 16.1, p<.0001). Only 9 of 80 (11%) demonstrated an ischemic ST segment response to exercise. Over a mean followup of 4.6 years; only 8 cardiac events have occurred versus 10 events in 80 age- and sex-matched control subjects without such complex exercise-enduced VEB (p = NS). C. The prognostic significance of 24-hr ambulatory ECG recordings was assessed in 100 healthy BLSA volunteers > 60 years old. Over a mean followup of 10 years, coronary events (CE) developed in 10 subjects. The prevalence and complexity of both supraventricular (SV) and ventricular (V) ectopic beats were similar in the groups with and without CE. However, CE occurred in 2 of 5 subjects (40%) with flat or downsloping ST segment depression > or = 1.0 mm versus only 8 of 95 (8%) without such ST changes.